The present invention relates to an imaging technique mainly used in medical diagnosis and, more particularly, to a method and apparatus for planning image scanning so as to obtain any tomographic image in any CT (computed tomograph) system, e.g., an MRI (magnetic resonance imaging) system, an X-ray CT system, an emission CT system, and an ultrasonic CT system for forming an image by utilizing an MR (magnetic resonance) phenomenon, radiation or ultrasonic waves in accordance with CT techniques.
A new age for medical diagnosis has been ushered in by the development and progress of X-ray CT systems. In a conventional X-ray (fluoroscopic) picture, one-dimensional information along the X-ray radiation direction is superposed to obtain a two-dimensional image (images corresponding to the depth of an examined portion cannot be obtained) in which the resolution of the one-dimensional image has been substantially lost. However, in an X-ray CT image, a minimum range in depth of an examined object represented by a two-dimensional image can be set to provide a slice, thereby obtaining an image having a one-dimensional resolution along the direction of depth. In earlier developed X-ray CT systems, only a cross-sectional tomographic image is obtained. However, in recent X-ray CT systems, any tomographic image at any angle with respect to the object can be obtained by changing the relative angle between the gantry (X-ray scanning unit) and an object to be examined.
The X-ray CT image is based upon examination of the coefficient X-ray absorption. In addition to the X-ray CT image, an MRI system has been recently developed which can obtain an image based on MR phenomenon information. In MRI systems of the present, slices at any direction can be obtained, and further developments are expected in the future.
As a CT application, an emission CT is proposed wherein image reconstruction is performed in accordance with transmission/absorption information obtained such that radiation is emitted from a radiation source such as a radioisotope to the object to be examined, and that the radiation is detected around the object. Another CT application is exemplified by an ultrasonic CT technique wherein transmission/absorption information obtained by projecting an ultrasonic wave is used to reconstruct a CT image. These CT techniques have been developed and applied in practice.
A scan planning system is proposed as a conventional X-ray CT system to photograph a slice at a proper position. A typical example of the scan planning system is described in U.S. Pat. No. 4,174,481. In such an X-ray CT scan planning system, an X-ray tube and an X-ray detector are moved together along an axis of an object to be examined to obtain a so-called scano-image which consists of one-directional projection images to form a two-dimensional image of the object. An operator designates a desired slice position with respect to the scano-image. Photographing (projection data acquisition) is performed at the designated position.
However, this system is based upon a CT system for obtaining slice images which are parallel to each other and which are perpendicular to the axis of the object. Therefore, a slice cannot be obtained at any angle and any position. (A tilted slice cannot be designated in the scano-image along the projection direction).
There is no scan planning system for obtaining any slice in the same manner as in the conventional MRI system.
When an image is scanned in the MRI system, a portion subjected to image scanning is roughly determined and several images are obtained in the vicinity of the portion at different conditions. Thereafter, a desired image is selected among a plurality of resultant images. As a result, many pictures are wasted, and repeated photographing imposes a mental and physical strain on the patient being examined.
In the scan planning system described in U.S. Pat. No. 4,174,481, a cross-sectional slice is designated. In this case, in order to obtain a tomographic image of a slice at any angle and any position in the X-ray CT system, a plurality of tomographic images are obtained and are subjected to image processing for image conversion. A desired tomographic image at a desired direction is reconstructed by interpolation. Since the resultant image is reconstructed by interpolation, its resolution is inevitably degraded. Furthermore, in the MRI system, in order to obtain a scano-image as a DR (digital radiography) constituting an image such as a normal X-ray image, a plurality of slice data must be obtained, and the scano-image is constructed by reproduction in accordance with the slice data even in a simplest case. In addition, since any slice can be photographed in the MRI system, a proper slice cannot be selected in an image such as a scano-image having a predetermined direction. Therefore, in order to accurately select a proper slice, an optimal slice is desirable determined in accordance with a plurality of images taken from different angles.